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I actually want to prevent this, and the second insertion should fail for duplicated key. Anyway, if I want to modify the value of any other column which is not primary key, for example from 'value' to 'VaLuE', I want to be able to do that, preserving the case insensitivity constraint only on the primary key.

This is possible on Oracle by creating an upper/lower case index on the table:

CREATEINDEXmyindexONt1(UPPER(mykey));

Anyway this doesn't seem to work on Sybase, since the duplicated-case insertion is still allowed.

I've found some hints around the web which would act on the DBMS configuration, through sp_configure/sp_text_configure, but I want to avoid such an intrusive solution, and apply the case sensitivity ONLY to some specific tables and ONLY on the primary keys of those tables, like I do on Oracle by creating the index through the syntax shown above.